A deep-dive beneath the mass-metallicity relation: Unveiling chemical enrichment in galaxies through infrared emission lines

Future and ongoing infrared (IR) and radio observatories such as JWST, METIS, and ALMA are increasing the amount of rest-frame IR spectroscopic data for galaxies by several orders of magnitude. The studies of the chemical composition of the interstellar medium (ISM) can be hugely improved exploiting the data obtained in these spectral ranges. Among the advantages of the IR regime, it is less affected by temperature and dust extinction, traces higher ionic species, and can also provide robust estimations of the chemical abundance ratio N/O. For instance, in (Ultra)-Luminous Infrared Galaxies ([U]LIRGs), the IR regime peers through their dusty medium and allows us to include the obscured metals in their studies. In this contribution, we describe how to take advantage of the bayesian-like code HII- CHI-Mistry-IR to analyze the chemical content in a sample of ULIRGs, reviewing our findings from the study of their IR emission, comparing them to the results from optical data, and ending up by describing the finding of deviations in a sub- sample of the analyzed ULIRGs from their expected mass- metallicity relation (MZR). We interpret this result as a consequence of the action of massive inflows of metal poor gas that produces that some galaxies experience a ``deep-diving’’ phase in the MZR diagram, as the metals from their ISM are diluted.